In multipoint conferencing, three or more endpoint terminals communicate with each other across a network. In the current telecommunications environment, multipoint conferencing is usually implemented over circuit-switched communication networks. Since connections are point-to-point in a circuit-switched network, a centralized resource, known as a multipoint control unit (MCU), is needed to link the multiple endpoints together. The MCU performs this linking by receiving multimedia (audio, video and/or data) information signals from endpoint terminals over point-to-point connections, processing the received information signals, and retransmitting the processed signals to selected endpoint terminals in the conference.
With the emergence of packet-based networks including local area networks, wide area networks and the Internet, multipoint conferencing is currently being defined for use over these packet-based networks. In packet-based networks, information is divided into blocks of data known as packets. Packets can be transmitted among endpoints using either unicast (i.e., one source to one destination) or multicast (one source to many destinations) transmission based on header information which contains appropriate unicast or multicast addressing information. Multicast is a technique that allows copies of a single packet to be transmitted to a selected subset of all possible destinations.
There are two methods known for multipoint conferencing using packet-based networks. These methods are referred to as xe2x80x9ccentralizedxe2x80x9d and xe2x80x9cde-centralizedxe2x80x9d modes, respectively, in the draft ITU standard H.323 (ITU-T Draft Recommendation H.323: xe2x80x9cVisual Telephone Systems and Equipment for Local Area Networks Which Provide a Non-Guaranteed Quality of Servicexe2x80x9d, Jan. 30, 1996 incorporated herein by reference). In the centralized mode, endpoint terminals in a conference communicate with a centralized resource using unicast transmission. The centralized resource can be an MCU that provides control and processing functions to endpoint terminals in the conference. The MCU processing functions include mixing and switching of multimedia (audio, video and/or data) information streams received from the endpoints. In the centralized mode, the MCU unicasts these processed information streams to each endpoint terminal.
In the de-centralized mode, endpoint terminals multicast their multimedia information streams to all other endpoint terminals, rather than through an MCU. Each endpoint terminal is then responsible for selecting among the incoming streams and performing its own audio and video processing functions. If an MCU is included in a de-centralized system, it is used as a bridge between the multicast environment and a separate unicast environment.
The unicast nature of the centralized mode is bandwidth inefficient relative to the de-centralized mode. That is, for N endpoints, there are 2N unicast streams transmitted in the centralized mode: N streams sent toward the MCU and N streams sent from the MCU. Moreover, most of the N streams unicast from the MCU are likely to contain the same audio or video information. For the de-centralized mode without an MCU, there are only N streams transmitted among the endpoints using multicast transmission. However, as the number of endpoint terminals in a de-centralized conference grows, the amount of streams increases, requiring greater and faster processing at the endpoints. A need exists for a conferencing mode that provides the benefits of centralized processing and the bandwidth efficiency of multicast transmission.
The above and other problems are solved by the method and apparatus of the present invention. The present invention provides a centralized multipoint conferencing arrangement which uses a combination of multicast and unicast transmissions that is bandwidth efficient.
Accordingly, a method of conferencing includes transmitting multimedia streams from endpoint terminals to a central resource using unicast transmission. The multimedia streams are processed in the central resource and transmitted back to the endpoint terminals using multicast transmission. In addition, the central resource transmits other multimedia streams to selected endpoint terminals using unicast transmission. At the selected endpoint terminals, processing of the multicast multimedia streams is inhibited in favor of the unicast streams. In one embodiment, the selected endpoint terminals ignore the multicast multimedia streams upon receiving the unicast streams. In another embodiment, the central resource transmits a control command to the selected endpoint terminals to inhibit processing of the multicast streams.
According to one aspect of the invention, a selected endpoint terminal associated with a designated video broadcaster in a conference receives a unicast video stream in addition to the multicast streams.
According to another aspect of the invention, a selected endpoint terminal associated with a designated audio broadcaster in a conference receives a unicast audio stream in addition to the multicast streams.